Field of the Invention (Technical Field)
The present invention relates to an optical tomographic imaging apparatus, and in particular, to an optical tomographic imaging apparatus that takes a tomographic image according to OCT used in ophthalmological treatment.
Description of Related Art
Because of noninvasive and contactless characteristics, the optical coherence tomography (OCT) has been widely used as a means for acquiring high-resolution tomographic images of living tissues in the department of ophthalmology.
The optical coherence tomography (OCT) is classified into time domain OCT called as a time domain method that acquires a tomographic image while moving a mirror to mechanically change an optical path length of reference beam, spectrum domain OCT called as Fourier domain method that detects spectrum information with a spectroscope to acquire a tomographic image, and optical frequency sweep OCT that detects a spectrum interference signal using a wavelength scanning light source to acquire a tomographic image.
Birefringence that changes polarization state occurs in tissues in which molecules are arrayed in a certain direction. In the retina on the fundus oculi, retina nerve fiber layer, pigmented layer of retina, vessel wall, sclera, cribrosa lamina have strong birefringence. The polarization sensitive OCT (PS-OCT) as one type of the functional OCT can visualize these tissues through tomography using birefringence. Thus, various polarization sensitive OCTs have been recently developed.
The polarization sensitive OCT (PS-OCT) uses circularly polarized light or polarization-modulated light as a measuring beam for observing a sample, and detects coherence light as two orthogonal linear polarized beams.
Japanese Patent No. 4344829 discloses an example of the polarization sensitive OCT (PS-OCT). According to this literature, simultaneously with (in synchronization with) B scan, a polarized beam from a light source (beam linearly polarized by a polarizer) is continuously modulated using an EO modulator (polarization modulator, electro-optic modulator). The continuously modulated polarized beam is divided, one beam is projected to a sample to acquire reflected light, and the other beam is used as reference beam. In OCT measurement based on spectrum interference of the beams, out of the spectrum interference components, a vertical polarization component and a horizontal polarization component are simultaneously measured using two optical detectors to obtain a Jones matrix indicating polarization characteristics of the sample.
According to a method disclosed in Lim et al., “Birefringence measurement of cornea and anterior segment by office-based polarization-sensitive optical coherence tomography”BIOMEDICAL OPTICS EXPRESS, Vol. 2, No. 8, Aug. 1, 2011, 2392, to improve the SNR (signal noise ratio) and quality of the image corresponding to the Jones matrix obtained by use of the polarization sensitive OCT (PS-OCT), a B scan image corresponding to the Jones matrix is subjected to moving average processing, for example, with a kernel size of 3×5 pixels.
When the B scan image corresponding to the Jones matrix is subjected to the moving average processing with a predetermined kernel size as in Lim et al., to reduce speckle noise, the method of finding a global phase difference of each pixel in the kernel and cancelling the global phase difference has been proposed.
However, since phase change caused by birefringence of a test sample (hereinafter referred to as “sample”) cannot be distinguished from global phase, the birefringence of the sample may become an artifact for the global phase.
To solve the above-mentioned problem, an object of the present invention is to provide an optical tomographic imaging apparatus for acquiring a two-dimensional or/and three-dimensional tomographic image corresponding to a Jones matrix acquired by a polarization sensitive OCT (PS-OCT), the apparatus performing average processing using a predetermined kernel region, in which a phase change caused by birefringence of a sample is cancelled to calculate a global phase difference of each pixel in the kernel, to perform moving average processing, thereby acquiring a two-dimensional or/and three-dimensional tomographic image having higher SNR and quality.